Automatic telephone system



Nov. 1938. N. H. SAUNDERS ET AL 2,135,921

AUTOMATI C TELEPHONE SYSTEM Filed Sept. 12, 1955 CONN.

FINDER REPEATER 90 G1 IH IN VEN TORS Q L NORMAN H.SAUNDER$ RALPHR.FOWLER TO EXCHANGE B ATTORNEY.

1 Patented Nov. 8, 1938 UNITED STATES AUTOMATIC TELEPHONE SYSTEM NormanH. Saunders and Ralph R. Fowler, Chicago, 111.. assignors, by mesneassignments, to Associated Electric Laboratories, Inc., Chicago, 1ll., acorporation of Delaware Application September 12, 1935, Serial No.40,196

21 Claims.

The present invention relates in general to automatic telephone systems,and more particularly to such systems as comprise two or more exchangeswhich are interconnected by trunk 5 lines. The object of the invention,briefly stated, is to provide new and improved trunk circuits forinterconnecting two automatic exchanges.

A special feature of the invention is a new and improved repeater.

may be used at each end of a two way trunk line.

The repeater includes a number of improvements among which may bementioned those having to do with the two way operation of a trunk lineand others relating to the automatic transmission of standardized orcorrected im pulses.

The invention will be described hereinafter with reference to theaccompanying drawing, 20 which shows diagrammatically sumcient of theequipment and circuits involved to enable the invention to be explainedand understood.

Referring to the drawing, the equipment there illustrated is all locatedin one exchange, which may be the type shown in the Ostline Patent No.

25 2,099,429, granted Nov. 16, 1937, or in the Saunders Patent No.1,999,788, granted April 30, 1935. The exchanges described in thesedisclosures are small exchanges employing finder connector links forsetting up connections. The exchange might also be a relay exchange ofthe type disclosed in the Ostline Patent No. 2,023,239 granted Dec. 3,1935, and there are other exchanges to which the invention isapplicable.

The drawing shows one end of a trunk line interconnecting two exchangesof the above mentioned character. There may be a number of these trunklines, all the same as the one shown. The inter-exchange portion of thetrunk line shown comprises two conductors 4| and 42. At the end whichappears in the drawing the trunk line is provided with a repeater and aline circuit, both of which are so labeled in the drawing, and ismultipled in the banks of connector switches and finder switches." Thefinder and connector branches of the trunk line appear at the right ofthe drawing, which also indicates one finder and one connector havingaccess to the trunk line. The equipment at the other end of the trunkline may be exactly the same and for convenience it will be assumed thatsuch is the case.

In the following description of the operation, the exchange at which theequipment shown in the drawing is located will be referred to asexchange A, and the exchange at the other end of the trunk line will bereferred to as exchange B.

Assuming now that a subscriber in exchange A desires to call asubscriber in exchange B, he-

60 will remove his receiver, whereupon his line is One of these newrepeaters.

picked up by the finder switch of a finder connector link. 'Ihe callingsubscriber then dials the digit or digits assigned to exchange B, withthe result that the connector of the link in use will be operated toselect the group of trunk lines extending to that exchange, and willthen proceed to select an idle trunk line in the group. Assuming thatthe connector indicated in the drawing is used, and that the trunk lineshown is the trunk selected, when the connector wipers 24 engage bankcontacts 8l9 a ground will be placed on the test wiper 4. A circuit isthus completed by way of 4, I0, 81, and I8.to the cut-off relay H5 inthe line circuit. Upon energizing, relay l5 disconnects the line relayl4, and at 23 closes a circuit for relay iii. The operation of thelatter relay has no significance at present.

The completion of the circuit of the cut-off relay l5, as abovedescribed, also grounds conductors 25 and 26 which extend between theline circuit and the repeater, terminating at the repeater in relays 49and 39, respectively. Relays 49 and 39 therefore have their circuitsclosed simultaneously. Relay 39 is arranged to open the circuit of relay40 at 50, but is slow to energize, and it follows that relay 4!! pullsup and locks itself at 45 before relay 39 operates.

On energizing, relay 40 connects the trunk conductors 4i and 42 with theleft-hand windings of the repeating coil by way of 44 and 48. Since thetwo left-hand windings of the repeating coil are connected together byway of the right-hand winding of the electropolarized relay 31 andcontacts and 13, the right-hand winding of relay 31 is bridged acrossthe trunk conductors 4i and 42. Responsive to the closure of thisbridge, the line relay such as Id at exchange B operates, starts up afinder in exchange B, and makes the trunk line busy at that exchange. Asa result the trunk line is extended through in exchange B to theconnector of a finder connector link. These operations will be describedmore in detail and with reference to the drawing in the course of theexplanation of a call originating at exchange B and directed to asubscriber in exchange A.

As a further result of the operation of relay 40, circuits are closed at46 for the left-hand or polarizing winding of relay 31 and for relay 38.The direction of current flow over the trunk 4I-42 is such that relay 31does not operate. Relay 38 energizes and locks itself by way of 53 and5| as soon as relay 39 operates. In energized position relay 38disconnects the conductors 85 and 86 from conductors 4| and 42 of thetrunk and connects them through by way of contacts of the reversingrelay 36 to the right-hand windings of the repeating coil. Conductor 85is thus extended by way of 52, contacts controlled Upon energizing,relay 33 close; a forreiay 32 at 33. Relay 32, on energizing, locksitself at 33 so that it can stay up independent of ground at 32, andcloses a circuit for the slow acting release relay 33 at 33. Onenergizing, relay 33 closes another circuit for itself by way of 32 and33, and grounds conductor 23 at 34.

By the latter operation ground is extended back over 23, 31, I3, and l,to the release trunk conductor in the connector to maintain theconnectar and finder of the link circuit in operated position after thefalling back of the line and release relays of the connector. Theserelays deenergize following the switching through operation. Ground onconductor 23 also holds up the cut-off relay ii, and relays 33' and 33of the repeater.

When the calling subscriber dials the next digit of called number, thecalling line loop is interrupted a number of times depending on thevalue of the digit, and as a result theline relay 33 is deenergized ammiber of times to produce a like number of interruptions in the bridgeacross trimk conductors 4| and 42, and thus repeat the digit over thetrunk line to exchange 13. The repeating operation will now be describedin detail.

When line relay 33 deenergizes at the begin- ,ning of the firstinterruption it breaks the circuit of relay 32 at 53 and relay 32 fallsback. Relay 33 is slow to release and remains operated while relays 33and 32 are responding to dialling. On falling back relay 32 closes acircuit for the slow acting relay 2] by way of 32, 33, and 31. Relay 21energizes and stays operated until the digit is completed. In energizedposition, relay 2! closes a circuit for relay 23 by way of 32, 33, and3|. Relay 23 therefore energizes and stays up until after relay 2! fallsback.

The impulses, or interruptions, are repeated by relays 23, 33, and 3|,under control of relay 32. When relay 32 falls back it closes a circuitfor relay 33 by way of 32, 33, 13, contacts controlled by 63', andcontacts controlled by II. On energizing, relay 33 breaks the bridgeacross the trunk conductors II and 32 at 13, and at 12 closes a circuitfor relay 23. Relay 23, on energizing, breaks the bridge at 15 and at Hcloses a circuit for relay 3| in parallel with relay 33. On energizing,relay 3| locks itself at 1|, and breaks the circuit of relay 33. Relay33 falls back, closes a point in the bridge at 13, and breaks thecircuit of relay 29 at 12. Relay 23 falls back'and closes the bridge at15, thus ending the first interruption in the bridge across the trunk"-32.

In the meantime the line relay 33 pull; up again, recloses the circuit'of relay 33 at 33, and at 33 recloses the circuit of relay 32. Relay32, however, cannot energize until the relays 23, 33, and 3| haveproceeded with their automatic operation to the point where relay 3| haslocked itself and relay 33 has fallen back. At this stage, if the linerelay 33 is up, a circuit is completed for relay 32 by way of I2, 13,and 33, and relay 32 reenergizes, breaking the locking circuit of relay3| at 33. Relay 3| falls back, whereupon conditions are the same as theywere before dialling startedexcept that relays 21 and 23 are holding ondue to the slow release characteristics of relay 21. If the digitdialled is the digit'l no further interruptions will occur at themoment, and relay 21 will fall back, followed by the deenergization ofrelay 23. If the digit dialled is greater than 1, relay 21 will ofcourse be maintained operated by an additional impulse or impulses fromrelay 32.

The relays respond to the remaining interruptions of the digit the sameway. At the beginning of each interruption relays 33 and 32 fallback.The latter relay closes a circuit for relay 33, whereupon relays 33, 23,and 3| pull up successively and relay 3| locks. Following the operationof relay 3|, relays 33 and 23 fall back successively. At the end of eachinterruption relay 3| pulls up again, followed by the energization ofrelay 32 contingent on relay 33 having restored. It will be seen thatthe length of the repeated interruptions produced by opening the bridgeat I3 and 13 is independent of the length of the interruptions in thecircuit of line relay 33, and has a constant value determined by theoperating characteristics of relays 23, 33, and 3|. A repeatedinterruption.

starts with the opening of the bridge at 13 on the energization of relay33 and it lasts while re ay 23 is pulling up, relay 3| is pulling up,relay 33 is falling back, and relay 23 is falling back, the interruptionbeing ended when the bridge is closed at II by the deenergization ofrelay 23.

In view of the foregoing it will be understood that some care isnecessary to insure that the relays have the proper operating andrelease characteristics, and it will be desirable to give some valuesthat have been found to work well in practice. An example that may becited is a commercial repeater designed to produce repeatedinterruptions having a constant length or duration of .060 second. Withthis length of interruptionrand with a dial speed of 10 impulses, orinterruptions, per second, the ratio of the open periods to the closedperiods is as 60 to 40. That is, the repeater output during dialling iswhat is known as a 60 per cent break and a 40 per cent make. Since theopen period or break remains constant the ratio of make to break changeswith the dial speed, and excessive variations of dial speed are notpermissible. The range within which the repeater will worksatisfactorily is about 8 to 12 impulses per sec- The desired constantbreak of .060 second is secured by using the following values: Relays 33and 3| are each wound to a resistance or 210 ohms, and relay 23 to aresistance of 300 ohms. The non-inductive resistance 33 shunting relay3| is 2000 ohms, resistance 31' shunting relay 33 is 1000 ohms, andresistance 33' shunting relay 23 is 2000 ohms. While not material to thelength of break, it may be stated also that the line relay 33 has aresistance of 200 ohms and relay 32 a resistance of 210 ohms. The latterrelay is shunted by a resistance 33 of 2000 ohms. The battery voltage isassumed to be about 46 volts. With the above values standard adjustmentsmay be used which last for a long time,

that other sets of values could be used, a change in one relay beingcompensated for by a change at another.

Some further operations which take place during the repeating of a digitwill now be explained. At 88 relay 21 short-circuits the upper righthandwinding of the repeating coil, which improves the operation of relay 34.With the same object, the impedance element 35 is shunted by resistances84 and 85' at 83 and 19. The impedance element 35 functions to balancethe line circuit in a well-known manner during the talking periods andfor this reason is of the same impedance value as the impedance value ofthe line relay 34. The shorting out of an impedance such as 35 toimprove the response of the line relay in the other side of the line hasbeen done before, but in this repeater the mere short circuiting of theimpedance 35 would not work. If impedance 35 were short circuited in theusual way by relay 21 during dialling, the removal of the short circuitat the end of a digit would suddenly introduce the impedance into thecircuit of line relay 34, causing the current to fall momentarily andproducing a slight or transient deenergization of the line relay. A veryshort deenergization such as this, only a small fraction of the lengthof a standard interruption, does no harm in an ordi-v nary repeater; butin this repeater only a very momentary falling back of relay 34 issufiicient to deenergize relay 32, which initiates a series ofoperations resulting in the transmission of a standard length impulse.To avoid any possibility of a false deenergization of line relay 34,therefore, relays 21 and 28 are arranged to remove the shunt fromimpedance 35 in two stages. During the dialling of a digit, impedance 35is shunted by resistances 84 and 85' which may be 50 and '100 ohms,respectively. When the digit is completed, relay 21 falls back andremoves the 50 ohm shunt of resistance 84. An instant later relay 28falls back and takes off the 700 ohm shunt of resistance 85. Thisarrangement has been found to be suificient to prevent any falsedeenergization of relay 34 under ordinary conditions. For more severeconditions, it is apparent that another slow acting relay could be addedto secure removal of the shunt in three stages instead of two.

Other operations which may be mentioned are the shunting of theright-hand winding of relay 31 at 18 during dialling, and the use of apriming circuit for line relay 34 to assist this relay in energizingquickly during dialling. The object of the first mentioned operationwill be obvious. As to the second, it will be noticed that whenever linerelay 34 falls back during the dialling it connects itself in serieswith relay 32 at 58, a resistance 98' being included in the circuit.-Resistance 98' may have a value of 3000 ohms and relays 34 and 32 cannotoperate in series with this resistance. The current flow established inrelay 34, however, primes the relay and enables it to respond morereadily when the line circuit is closed after each interruption.

Continuing now with the general explanation, the digits of the callednumber are repeated in the manner described and result in the completionof the connection to the desired subscribers line in exchange B. Whenthe called subscriber answers, the connector in use reverses thedirection of current flow over the trunk line 4I- 42 in the usualmanner, causing the operation of the electro-polarized relay 31. Onoperating, relay 31 closes a circuit for reversing' relay 38, whichreverses the direction of current flow in the calling subscriber's line.At this point it may be mentioned that if the calling party shouldaccidentally operates or move his dial so as to send one or moreimpulses, which would cause relay 31 to fall back due to being shortcircuited, the energized position of reversing relay 36 is not disturbedbecause of the provision of a locking circuit by way of 55 and 11.

When the calling subscriber hangs up his receiver, the release relay 33falls back following the continued deenergization of relays 34 and 32.Ground is thus removed from the holding circuit at 64, and the linkcircuit in use by the calling party is restored to normal in the usualmanner. When relay 32 in the repeater falls back, relays 21 and 28 ofcourse energize and relays 29, 38, and 3| go through their operations,but the interruption produced thereby in the bridge across the trunkdoes no harm. Relays 21, 28, and 3| are deenergized by the falling backof relay 33, which opens the circuits of the former relays at 52 and 83.The removal of ground from conductor 25 at 64 by the falling back ofrelay 33, also lets relays l and 48 fall back. The latter relay removesthe bridge from conductors 4| and 42 and initiates the release of theequipment in exchange B.

If the release of the connection should take place while current isreversed on trunk line 4l-42, no lock up can occur, due to thearrangement involving slow to release relays 39 and 38. When relay 33falls back, the line relay I4 is promptly connected up by the fallingback of relay l5, and if the trunk line were connected through at thistime the line relay of the distant connector would hold up in serieswith the line relay l4, energizing this latter relay also. Suchoperation is prevented because relay 48 falls back promptly on thefalling back of relay 33 and initiates the release at the distantexchange, while the slow acting relays 39 and 38 hold on for an intervaland hold the trunk line open at 52 and 54 long enough for the distantconnector to release and reverse the direction of current fiow back tonormal. The holding open of the trunk at 52 and 54 involves also themaintaining of the incoming line conductors connected through to therepeating coil by way of contacts of the reversing relay 36. Sincerelays 31 and 36 are energized when relay 48 falls back, assuming therelease takes place while current on trunk line 4 l42 is reversed, andsince it takes an instantforrelays-31 and 36 to deenergize, contacts 58and 41 on relay 48 are provided to disconnect line relay 34 andimpedance 35 from the repeating coil and prevent any possibility of thefalse operation of relay 34 in series with relay l4.

In order to complete the explanation, the operation of the repeater andline circuit on a call from exchange B to exchange A will be considered.

When the trunk line is seized at exchange B, the repeater on that end-of the trunk places a bridge across the trunk line just as explained inthe discussion of the previous call. The bridging of the trunk lineresults in the operation of the line relay l4 at exchange A. Uponenergizing, relay l4 grounds conductors 26 and 81 at 18, the ground on81 serving to make the trunk line busy in the banks of the connectors.Ground on 26 operates relay 39, which opens the circuit of relay 48 at58. It will be noted that on a call coming from exchange B conductor isnot grounded immediately, so relay 38 prevents relay 40 from operating.The line relay H also connects at I1 the cut-ofi relay I5 to testcontacts such as IS in the banks of the finders, thus placing a testpotential on these contacts, and at I8 closes a finder start circuitincluding conductor 90. An idle finder is then started up in knownmanner in search of the trunk line. Assuming the finder in use is theone indicated in the drawing, when the test wiper 1 reaches test contactII, the findertest relay energizes in series with cut-oi! relay l5 andstops the finder. Relay l5 locks itself at 2|, removes the shuntresistance at 22, and disconnects the line relay ll, which latter relaynow falls back. The cut-ofl relay I5 is held energized by ground fromthe finder and later from the connector of the link, the circuitincluding the finder test wiper I. These circuits are known and arefully explained in the disclosures hereinbei'ore referred to. It will benoted that as soon as the finder connects with the trunk line theconductor 25 will become grounded but by this time the relay 39 hasenergized andhence relay 0 cannot pull up.

It will be understood therefore that on. an incoming call from exchangeB the repeater at exchange A is not used. The trunk conductors 4| and 42continue through the repeater to the banks of the finders, and after afinder has picked up the trunk line, to the line relay oi the associatedconnector- The connection is completed by operating the connectorresponsive to the digits repeated over the trunk line by the distantrepeater in the manner previously explained.

The invention having been described, that which is believed to be newand for which the protection oi. Letters Patent is desired will bepointed out in the appended claims.

What is claimed is:

1. In a telephone system, two exchanges, finder and connector switchesin each exchange, a two-way trunk line connecting said exchanges, arepeater associated with said trunk line at each end thereof, and twobranches of said trunk line at each end, one branch accessible toconnector switches to extend an outgoing connection from an exchange viathe associated branch repeater and the other branch accessible to finderswitches to find the trunk line over which an outgoing connection hasbeen extended from another exchange.

2. In a telephone system, an exchange including finder and connectorswitches, a trunk line extending to another exchange, a first branchover which said trunk line may be seized by a connector to extend anoutgoing connection to the other exchange, a repeater operable to repeatimpulses when the trunk line is seized by a connector, a second branchof said trunk line accessible to said finders when an incoming connec--tion is received, line circuit equipment controlled over the trunk linewhen an incoming call is received from the other exchange to cause afinder .to find and seize said second branch, and means in the-repeaterfor discriminating between outgoing and incoming calls to connect thesame into the trunk line only on outgoing calls.

3. In a telephone system an exchange comprising finder and connectorswitches, a two-way trunk line connecting said exchange with anotherexchange, a repeater, means responsive to the seizure of said trunk lineby a connector for inserting said repeater therein to provide a holdingcircuit for said connector, a line relay energized over said trunk linewhen the same is seized at the other exchange, means controlled by saidline relay for starting a finder switch in search of the trunk line, andmeans controlled by said line relay for preventing the insertion of saidrepeater.

4. In a telephone system, an inter-exchange trunk line, a repeater, twoconductors normally connected together, a quick-acting relay energizedover one conductor and a slow-acting relay energized over the other,said slow relay having means to break the circuit of the quick relay,means for grounding said conductors simultaneously. when the trunk lineis seized at one end, whereby the quick relay is energized, meanswhereby said quick relay locks itself and connects the repeater in thetrunk line, and means for grounding the conductor associated with theslow relay while the other conductor is free of ground when the trunkline is seized at the other end, whereby the slow acting relay iseffective to prevent the operation of the quick relay and the connectionof the repeater in the trunk line.

5. In a repeater, a line relay and an impedance coil bridged a cross theincoming line in series with a battery, contacts on the line relay forrepeating impulses over the outgoing line, means for shunting the saidimpedance coil during the operation of said line relay in repeating aseries of impulses, and means for removing the shunt at the end of aseries in a plurality of stages, whereby the momentary falsedeenergization of said line relay which might result if the entire shuntwere removed at once is effectively prevented.

6. In a telephone system, two exchanges, a two-way trunk line connectingsaid exchanges, a repeater at one exchange including a repeating coil, arelay in said repeater responsive to the seizure of said trunk line tobridge one side of said repeating coil across the trunk line, a linerelay in the other exchange operated by the closure of the bridge, and abusy test circuit completed by the operation of said line relay, wherebyupon seizure of the trunk line it is made busy at the other end by thesuccessive operation of only two relays, one in the repeater atthe endseized and the other being the line relay at the other end.

'7. In a repeater, means responsive to the seizure of the repeater forclosing a bridge across the outgoing line, a line relay energized overthe incoming line, a series of relays responsive to the deenergizationof said line relay, and circuit arrangements whereby said series ofrelays open said bridge for a predetermined interval which isindependent of the length of time said line relay is deenergized.

8. In a repeater, a line relay, a series of three other relays, circuitsfor automatically and successively energizing said other relays and forsuccessively deenergizing two of said other relays responsive to themomentary deenergization 01. said line relay, and an impulse bridge heldopen by contacts on said two relays while the same are energized.

9. In a repeater, a line relay, a plurality of other relays, meansresponsive to the momentary deenergization of said line relay forautomatically energizing a plurality of said other relays in successionand for deenergizingthem in the same order, and an impulse repeatingbridge including contacts on one oi! said plurality of relays.

10. In a repeater, a line relay, a second relay initially responsive tosaid line relay u n re of the repeater, an outgoing trunk line,automatic impulse repeating means for transmitting impulses over saidtrunk line controlled by said second relay, and circuits effectiveduring the repeating of a digit for making the response of said secondrelay to said line relay contingent o the operation of said repeatingmeans.

11. In a repeater, a line relay, a second relay energized by said linerelay when the same responds to the seizure of the repeater, said secondrelay deenergizing when said line relay falls back at the beginning ofan interruption, repeating means responsive to the deenergization ofsaid second relay for producing an interruption of predeterminedduration in the outgoing line, and circuit arrangements for preventingthe reenergization of said second relay responsive to the reenergizationoi. the line relay until the operation of said repeating means hasreached a definite stage.

12. In a repeater, a line relay, a second relay energized by said linerelay when the same responds to the seizure of the repeater, said secondrelay deenergizing when said line relay falls back at the beginning ofan interruption, repeating means responsive to the deenergization ofsaid second relay for producing an interruption of predeterminedduration in the outgoing line, circuit arrangements for preventing thereenergization of said second relay responsive to the reenergization oi'the line relay until the operation of said repeating means has reached adefinite stage, and means for preventing said repeating means from fullycompleting its cycle of operation until said second relay has becomereenergized.-

13. In a repeater,- a line relay, a second relay energized by said linerelay upon the seizure of the repeater, a release relay initiallyenergized by said second relay, and a circuit closed by said releaserelay over which the same is subsequently controlled by said line relay.

14. In combination, a repeater, a trunk line normally extending throughsaid repeater, means responsive to the seizure of the trunk line foropening the trunk line at the repeater, for connect ng a line relaybridge across the trunk line on the seized side of the break, and forconnecting an impulse repeating bridge across the trunk line beyond thebreak, means for promptly opening said repeating bridge upon the releaseoi. the trunk line by the seizing means, and means for maintaining theopen condition of the trunk line at the repeater for a predeterminedtime after the repeating bridge is opened.

15. In a repeater, a line relay, a plurality of other relays, meansresponsive to the deenergization of said line relay for automaticallyand successively energizing said other relays and for deenergizing themin the same order, and an impulse repeating bridge including seriallyrelated contacts on two of said other relays.

16. In a repeater, a line relay responsive to digital series ofinterruptions, means including a pair. of auxiliary relays controlled bysaid line relay for repeating said interruptions, contacts on one 01'said auxiliary relays for beginning each repeated interruption andcontacts on the other auxiliary relay for ending each repeatedinterruption.

17. In a telephone system, two exchanges, a

two-way trunk line extending between said exchanges, a repeaterassociated with said trunk line at each end thereof, connector switchesin each of said exchanges individually operative to select said line toextend a call thereover, finder switches in each of said exchangesindividually operative to search for and seize said line when a call isreceived thereover, and means for selectively rendering the repeater atthe exchange receiving the call inoperative.

18. In a'telephone system, two exchanges, a two-way trunk line extendingbetween said exchanges, a repeater associated with said trunk line ateach end thereof, connector switches in each of said exchangesindividually operative to select said line to extend a call thereover,finder switches in each of said exchanges individually operative tosearch for and seize said line when a call is received thereover, andmeans controlled by the finder that seizes said line for selectivelydisconnecting the repeater at the exchange receiving the call from thecircuit of said line.

19. In a telephone system, two exchanges, a two-way trunk lineextendingbetween said exchanges, a repeater associated with said trunk line ateach end thereof, connector switches in each.

of said exchanges individually operative to select said line to extend acall thereover, finder switches in each of said exchanges individuallyoperative to search for and seize said line when a call is receivedthereover, means associated with the finder switch operated to seizesaid line for preventing the connector switches at the same exchangefrom connecting with the line to extend a call thereover, and meanscontrolled by the operated finder switch for rendering the repeaterconnected to the trunk line at the exchange receiving the callinoperative.

20. In a repeater, incoming and outgoing lines, a line relay, a batteryand a line balancing impedance element, said selector, battery, andimpedance all connected in series and bridged across said incoming line,said line relay including means for repeating impulses over saidoutgoing line, means for shunting said impedance element during theoperation of said line relay in repeating a series of impulses, saidimpedance element so connected that it would cause false operation ofsaid line relay from one of its two positions to the other it saidshunting means were removed in its entirety at the same instant, andmeans for removing said shunting means in a plurality of stages toprevent such false operation.

21. A repeater adapted to interconnect a pair of line circuits totransmit control impulses therebetween comprising, a line relayresponsive to impulses received over one of said line circuits, aplurality of relays responsive to operation of said line relay from oneoi. its two positions to the other for repeating said impulses over theother of said line circuits, and control circuits for interconnectingsaid plurality of relays and said

